This invention relates generally to electrical impedance measurement instruments and in particular to impedance measurement instruments employing pulses to measure complex impedances.
A variety of electronic instruments exist for measuring electrical impedance between a pair of terminals of a one-port device under test (DUT). Impedance, expressed in units of ohms, defines the relationship of the electrical current I through the DUT to the voltage V across the DUT. In the simplest case, impedance may be purely resistive such that the voltage and current in the DUT are in phase. The relationship is governed by Ohm's law such that R=V/I where R is the resistance. Impedance may also be complex when there is a significant amount of reactance in the DUT from capacitive or inductive elements. Complex impedance is thus composed of the resistive element R and a reactive element X which causes the current through the DUT to lead or lag the voltage. Complex impedance Z is often expressed as the equation Z=R+jX. Complex impedances typically vary as a function of frequency and therefore the user must specify the frequency range of interest to obtain a meaningful complex impedance value. Thus, the impedance equation becomes Z(f)=R(f)+jX(f) where f is the frequency of interest.
Electronic measuring instruments capable of determining the complex impedance of one-port devices include vector network analyzers and electronic component analyzers. Vector network analyzers and component analyzers are closely related and operate in a similar manner, differing mainly in how the measured complex impedance information is processed and displayed. Vector network analyzers display complex impedance graphically, typically in the form of impedance versus frequency. Component analyzers, in addition to graphically displaying the impedance information, often use the impedance information to mathematically model electronic component parameters, such as the parasitic capacitance that occurs across a resistor, which is modeled as a shunt R-C circuit. Newer instrument designs now further blur the distinction between vector network analyzers and component analyzers by incorporating more features traditionally found in only one or the other type of instrument.
Both vector network analyzers and component analyzers impose an incident signal across the two terminals of the DUT in the form of a continuous wave (CW) signal at high frequencies. Because these analyzers employ linear receivers to receive the reflected signal over a selected range of frequencies, the incident signal is usually in the form of a swept-frequency sine wave with a high amount of spectral purity. The instrument then determines the reaction of the DUT to the incident signal by measuring the resulting reflected signal from the DUT and comparing it to the reflected signal from a reference resistor with a known impedance to arrive at a return loss measurement, which is in terms of both magnitude and phase. From the return loss measurement, the impedance between the two terminals of the DUT may be calculated. Because vector network analyzers and component analyzers typically provide swept-frequency sine wave measurements, a return loss versus frequency or impedance versus frequency graph may be obtained by specifying the frequency range of interest.
While capable of providing superior accuracy in measuring impedance and return loss, vector network analyzers tend to be expensive, complex, and bulky. Vector network analyzers are therefore limited to laboratory and bench-top applications rather than field service applications. Furthermore, only a very sophisticated subset of available vector network analyzers provide an ability to extract distance information using mathematical transforms. The typical network analyzer provides only a limited ability to troubleshoot and locate the possible causes of an impedance or return loss measurement along a transmission line DUT that fails a performance specification limit.
Pulse-based measurements of impedance may be performed by a time delay reflectometer (TDR) in a manner well known in the art. The TDR performs an impedance measurement by introducing an incident pulse of known magnitude into the DUT and measuring the resulting reflected signal. Because the incident pulse width can be made very narrow, typically less than ten nanoseconds, the TDR can measure impedance as a function of time along the transmission line with high resolution. TDR's thus have the ability to troubleshoot transmission lines by detecting discontinuities that can disrupt signals and are most often applied in measuring the impedance along transmission lines which include coaxial cables and twisted-pairs. Measuring impedance at selected points along the transmission line has the advantage of allowing faults or discontinuities along the transmission line to be detected and localized, a feature particularly desirable for field service applications. If the propagation velocity of the signals through the transmission line are known, the time delay between incident and reflected pulses may be used to determine the distance to the fault from the instrument along the transmission line. Simple, inexpensive, and portable TDR's for field service applications are commercially available to perform such measurements.
In performing an impedance measurement with a TDR, the magnitude of the reflected pulse as a fraction of the incident pulse may be used to calculate the characteristic impedance at any given point along the transmission line as referenced to the output impedance of the TDR. In most cases, a TDR measurement is adequate to provide a characteristic impedance of a transmission line and to locate discontinuities along the transmission line. However, TDR measurements provide only magnitude versus time information using analog techniques. As a pulse is launched, an analog trace is swept along a horizontal display, deflected vertically by the voltage level of the reflected signal. Such traditional TDR techniques do not measure impedance versus frequency of the DUT, but rather display only its pulse response. Therefore, it would be desirable to provide a low cost, portable, pulse-based impedance measurement instrument that measures complex impedance and return loss.